1. Field of the Invention
The invention relates to catalyst rejuvenation in a slurry. More particularly the invention comprises rejuvenating catalyst particles in a three-phase slurry body, by sequentially passing slurry from the top down to the bottom of the body through a first gas disengaging zone, a rejuvenating gas contacting zone, a second gas disengaging zone and then a downcomer, and a means for achieving this. This is useful for rejuvenating a Fischer-Tropsch hydrocarbon synthesis catalyst in-situ in a slurry hydrocarbon synthesis reactor.
2. Background of the Invention
Slurry hydrocarbon synthesis processes are well known and documented. In a slurry hydrocarbon synthesis process, synthesis gas comprising a mixture of H2 and CO is bubbled up as feed gas through a three-phase, gas-liquid-solids slurry in a hydrocarbon synthesis reactor. The slurry liquid comprises hydrocarbon products of the synthesis reaction, the solids comprise a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst and the gas bubbles comprise the feed gas and gaseous products of the synthesis reaction. Synthesis gas made from hydrocarbon feedstocks which contain nitrogen (i.e., natural gas) or nitrogen containing compounds (i.e., resids, coal, shale, coke, tar sands, etc.) invariably contains HCN and NH3, either of which deactivates the catalyst. Some compounds formed in the slurry as by-products of the synthesis reaction, such as oxygenates, also cause reversible deactivation. deactivation by these species is reversible and catalytic activity is restored (the catalyst rejuvenated) by contacting the deactivated catalyst with a hydrogen rejuvenating gas. Thus, the activity of the hydrocarbon synthesis catalyst in the reactive slurry may be intermittently or continuously rejuvenated by contacting the slurry with hydrogen or a hydrogen containing gas, to form a catalyst rejuvenated slurry. This is disclosed, for example, in U.S. Pat. Nos. 5,260,239; 5,268,344; 5,817,702 and 5,811,468. In the ""239 and ""270 patents, rejuvenation takes place in an external vessel. In the ""702 and ""468 patents, slurry containing the rejuvenated catalyst is passed up towards the top of the reactor, where the feed gas concentration is low. It would be advantageous to be able to take the slurry to be rejuvenated from near the top of the reaction vessel, rejuvenate the catalyst inside, instead of in a vessel external of the reactor and then pass the rejuvenated slurry down near to the bottom of the vessel, to contact the incoming feed gas.
The invention relates to a process and means for slurry in-situ catalyst rejuvenation, in which slurry is passed from the top to the bottom of the slurry body in a reactor, through a rejuvenating zone and means located in the slurry body. The process rejuvenates the catalyst in the slurry in, and not external to the reactor, thereby eliminating the need for an external rejuvenation vessel. The invention may be practiced with any process employing a three-phase slurry in which the catalyst reversibly deactivates and is rejuvenated with a suitable rejuvenating gas. An example of a particularly suitable process is a slurry Fischer-Tropsch hydrocarbon synthesis process, in which the slurry comprises a particulate catalyst and gas bubbles dispersed in a hydrocarbon slurry liquid. In a slurry hydrocarbon synthesis process, the feed gas concentration is greatest at the bottom of the slurry (slurry body) in the reactor. Therefore, it is advantageous to remove the deactivated catalyst from near to the top of the slurry body where the concentration of the feed gas is lowest and pass the rejuvenated catalyst down to the bottom portion of the reactor. By slurry body is meant the body of three-phase slurry in reactor. By top and bottom of the slurry body, is respectively meant above and below the middle of the slurry body, and preferably as near to the top and bottom as possible.
Briefly, the process comprises rejuvenating reversibly deactivated catalyst particles in a three-phase slurry body, by sequentially passing slurry from the top down to the bottom of the body through a first gas disengaging zone, a rejuvenating gas contacting zone, a second gas disengaging zone and then a downcomer or transfer zone down, all of which occurs within the slurry body. Further, except for the first gas disengaging zone and the bottom exit of the downcomer transfer zone, the slurry being processed according to the invention is isolated from direct contact with the surrounding slurry body. Having the rejuvenation zone and the second gas disengaging and downcomer zones isolated from direct contact with the surrounding slurry body, enables the slurry to be rejuvenated by the process of the invention while the reactor is either off-line or on-line producing products. The rejuvenation process may be conducted continuously or intermittently in the reactor. At least a portion, but not necessarily all, of the gas bubbles in the slurry are removed in the gas disengaging zones. The first gas disengaging zone removes gas bubbles from the slurry as it passes through the zone, to produces a gas reduced slurry that passes into the rejuvenating zone, into which a rejuvenating gas is passed and contacts the catalyst particles in the gas reduced slurry. This rejuvenates the catalyst in the slurry and forms a rejuvenated slurry containing bubbles of rejuvenating gas, which is then passed through a second gas disengaging zone, to remove bubbles of rejuvenating gas and form a gas reduced and rejuvenated slurry, which is passed down to the bottom of the slurry body. By rejuvenated slurry is meant slurry in which the catalyst particles have been at least partially rejuvenated. By removing bubbles is meant that at least a portion of the gas bubbles is removed, as is explained below. Removing gas bubbles from the slurry increases its density, compared to the lower density of the higher gas content surrounding slurry body and slurry being rejuvenated in the rejuvenating zone. This density difference enables the rejuvenation process to be conducted entirely by gravity driven hydraulics. Thus, there is no need for slurry pumps.
Means useful for rejuvenating a reversibly deactivated catalyst according to the process of the invention conducting the process of the invention may comprise a simple closed vessel at least partially immersed in the slurry body, having a slurry entrance and exit, and means for injecting rejuvenating gas up into its interior. The entrance and exits are in direct fluid communication with the first and second gas disengaging means, each of which may comprise an upward opening cup or conduit that permits gas bubbles to rise up and out of the slurry passing through and which respectively feeds the gas-reduced slurry into the interior of the vessel and into a downcomer. The downcomer may be a simple, vertical pipe or conduit open at its top and bottom, part of which is immersed in the slurry body and feeds the rejuvenated and gas-reduced slurry down into the bottom of the surrounding slurry and with its top extending up and out the top of the slurry body, to permit the disengaged rejuvenating gas bubbles leave the slurry, before or as it passes down the conduit.
The invention briefly comprises a process for rejuvenating reversibly deactivated catalyst particles in a three-phase slurry, wherein slurry from a slurry body is sequentially passed from the top down to the bottom of the body through a first gas bubble reducing zone, a rejuvenating gas contacting zone in which a catalyst rejuvenating gas contacts the catalyst which is at least partially rejuvenated, a second gas bubble reducing zone and then a downcomer or transfer zone, wherein the gas bubble reducing and contacting occur within the slurry body, wherein at least a portion of the transfer zone is in the body and wherein the slurry comprises gas bubbles and solid catalyst particles in a slurry liquid. The bubble reducing zones, the rejuvenating gas contacting zone and at least a portion of the downcomer transfer zone are part of a single catalyst rejuvenating means at least partially immersed in the slurry body. In the embodiment in which the single rejuvenating means is immersed in a slurry in a slurry reactor, then the all of the downcomer transfer zone will also be immersed in the slurry body in the reactor. In greater detail the invention comprises a process for rejuvenating a reversibly deactivated catalyst in a three phase slurry body in a vessel, wherein the slurry comprises a slurry liquid in which is dispersed gas bubbles and particles of the catalyst, the process comprising (i) passing slurry from the upper portion of the slurry body through a first gas bubble reducing zone, to disengage and remove at least a portion of the bubbles and form a gas bubble reduced slurry, (ii) passing the gas bubble reduced slurry into a rejuvenating zone, (iii) passing a rejuvenating gas into the rejuvenating zone, in which it contacts the catalyst particles in the slurry and reacts with them to rejuvenate at least a portion and form a rejuvenated slurry containing gas bubbles, (iv) passing the rejuvenated slurry through a second gas bubble reducing zone to remove gas bubbles and form a gas bubble reduced and rejuvenated slurry and (iv) passing the gas bubble reduced and rejuvenated slurry into a downcomer slurry transfer zone, in which the gas bubble reduced and rejuvenated slurry passes down and into the lower portion of the slurry body, and wherein the gas bubble reducing zones, the rejuvenating zone and at least a portion of the downcomer slurry transfer zone are located in the slurry body. In the specific case of a slurry Fischer-Tropsch hydrocarbon synthesis process, the rejuvenation gas will be hydrogen or a hydrogen-containing gas.
In an embodiment with specific regard to a slurry hydrocarbon synthesis process, the invention comprises the steps of:
(a) contacting a synthesis gas comprising a mixture of H2 and CO with a particulate hydrocarbon synthesis catalyst in the presence of catalyst deactivating species, in a slurry body comprising the catalyst and gas bubbles in a hydrocarbon slurry liquid, in which the H2 and CO react under reaction conditions effective to form hydrocarbons from said synthesis gas, at least a portion of which are liquid at the reaction conditions and form the slurry liquid, and wherein the deactivating species reversibly deactivate the catalyst;
(b) passing slurry from the upper portion of the slurry body through a first gas bubble reducing zone, to remove at least a portion of the gas bubbles from the slurry and form a gas bubble reduced slurry;
(c) passing the gas bubble reduced slurry into a catalyst rejuvenation zone;
(d) passing a catalyst rejuvenating gas comprising hydrogen into the rejuvenation zone in which it contacts and reacts with the catalyst particles in the slurry, to rejuvenate and at least partially restore the activity of the catalyst to form (i) a rejuvenating offgas and (ii) a rejuvenated catalyst slurry containing bubbles comprising offgas and unreacted rejuvenating gas;
(e) passing the rejuvenated catalyst slurry into a second gas bubble removing zone to remove at least a portion of the gas bubbles from the rejuvenated slurry to form a gas reduced and rejuvenated catalyst slurry, and
(f) passing the gas reduced and rejuvenated slurry into a downcomer slurry transfer zone in which it is passed down and into the bottom portion of said slurry body and wherein the gas bubble reducing zones, the rejuvenating zone and at least a portion of the downcomer slurry transfer zone are located in the slurry body.
The slurry reactor may be operating during rejuvenation or it may be taken off-line and batch rejuvenated. The rejuvenation steps (b) through (f) may be conducted either continuously or on a cyclical basis. When rejuvenation occurs while the hydrocarbon synthesis reactor is on-line and producing hydrocarbon liquids, a portion of these liquids is either continuously or intermittently withdrawn from the reactor as synthesized hydrocarbon product. In the context of the invention, the term xe2x80x9ccatalyst deactivating speciesxe2x80x9d is meant to include species which reversibly deactivate the catalyst and wherein the catalyst activity is restored (the catalyst rejuvenated) by contact with a rejuvenating gas in-situ in the slurry liquid. Hydrogen or a hydrogen containing gas is useful for such rejuvenation in a hydrocarbon synthesis process, as has been demonstrated in the prior art. Finally, while HCN, NH3 and certain types of oxygenates will deactivate a hydrocarbon synthesis catalyst, the invention is not intended to be limited to use only with a hydrocarbon synthesis process or a hydrogen rejuvenating gas, but is useful with any slurry reaction, catalyst and species which reversibly deactivate the catalyst and wherein the catalyst activity can be restored with an appropriate rejuvenating gas.